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Older people with a history of hypothyroidism may be at a much higher risk for developing dementia, a Taiwanese study found.

Among patients ages 65 and older, hypothyroidism was linked with an 81% increased risk of being diagnosed with dementia (adjusted OR 1.81, 95% CI 1.14-2.87, P=0.011), reported Chien-Hsiang Weng, MD, MPH, of Brown University in Providence, Rhode Island, and colleagues.

However, this association wasn't seen in patients ages 50 to 64 (adjusted OR 2.43, 95% CI 0.83-7.12, P=0.106), the group noted in .

Even after adjusting for history of stroke and transient ischemic attack, hypothyroidism remained associated with an increased risk for dementia in those 65 and older.

Certain clinical factors appeared to play a role in this association. For instance, patients 65 and older with a history of hypothyroidism who received medication for the condition saw the highest risk of developing dementia (adjusted OR 3.17, 95% CI 1.04-9.69, P=0.043).

"These are most likely the patients with overt hypothyroidism or symptomatic subclinical hypothyroidism," the researchers pointed out.

As for hyperthyroidism, there was no increased risk for dementia seen at any age or among those who received different types of hyperthyroidism treatment, including medication, surgery, or a combination of the two.

Furthermore, patients 65 and older who developed hypothyroidism after hyperthyroidism treatment also didn't appear to have an excess dementia risk.

"In some cases, thyroid disorders have been associated with dementia symptoms that can be reversible with treatment," Weng explained in a statement.

"While more studies are needed to confirm these findings, people should be aware of thyroid problems as a possible risk factor for dementia and therapies that could prevent or slow irreversible cognitive decline," he added.

For this case-control study, Weng and team included 7,843 individuals with newly diagnosed dementia in the Taiwanese National Health Insurance Research Database from 2006 to 2013. The control group consisted of 7,843 individuals without dementia.

Mean age of patients in both groups was 75 and 52% of participants were female. Significantly more patients with dementia had hypertension, coronary artery disease, depression, tinnitus, and alcohol dependence.

Thyroid disorders were identified using ICD-9 codes; 0.9% of patients with dementia had hypothyroidism and 1% had hyperthyroidism compared with 0.4% and 0.7% in the control group.

Patients with a dementia diagnosis with only one inpatient visit were excluded, as were those diagnosed with syphilis, HIV, Huntington's disease, Creutzfeldt-Jakob disease, cerebral degeneration, or Parkinson's disease before the dementia diagnosis.

Results were taken from logistic regression models and were adjusted for sex, age, history of hypertension, diabetes, coronary artery disease, depression, hyperlipidemia, alcohol dependence syndrome, tinnitus, hearing loss, and radioactive iodine treatment.

A limitation of the study is that it could not capture severity of thyroid disease or exact timing of disease development from the information in the claims database.

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